WERE it not for meteorites striking Earth some four billion years ago, humans would never have laid eyes on the gold that has raised and ruined civilisations, according to a study just published.

Two hundred million years earlier, during the violent throes of planetary formation, Earth was a mass of molten minerals set afire by collisions with planet-sized heavenly bodies.

These Olympian crashes probably threw up the Moon and also caused billions of tonnes of liquefied gold and platinum - enough to cover the planet with a crust four metres thick - to sink to centre, creating its core.

And there the precious metals lie, forever beyond the reach of grasping human hands.

This much was known. The enduring mystery was why there remained anything more than trace amounts of gold in Earth's outermost skin, called the crust, and the layer underneath, the mantle.

Indeed, precious metals are tens, perhaps thousands of times more abundant in Earth's silicate mantle than they should be if none were added after the great meltdown during the planet's early phase.

Hence the theory that gold and other shiny rare metals were imported from outer space after our orbiting, rotating home had cooled enough to harden, at least partially, into rock.

But it was only a theory.

To put it to the test, a trio of researchers led by Matthias Willbold of the University of Bristol in Britain analysed ancient rocks from Greenland that took shape shortly after the formation of the core but before the suspected bombardment of meteorites.

Their results were published in the peer-reviewed journal Nature.

Using cutting-edge technology, they measured the isotopic composition of tungsten, a very rare element which, like gold and other heavy precious metals, gravitated to Earth's centre during the formation of the core.

When atoms have the same chemical make-up but a varying number of neutrons, which changes an atom's mass, they are said to have different isotopes.

These minute variations can indicate both the origin and age of a mineral.

Comparing the Greenland samples to rocks from elsewhere on the planet, the researchers detected a tiny but unmistakable 15-parts-per-million difference in the abundance of a particular isotope, called 182W.

The gap is perfectly consistent with the theory that the "excess" gold which humans so covet is a fortuitous by-product of an ancient meteorite shower.

"Our work shows that most of the precious metals on which or economies and many key industrial processes are based have been added to our planet by lucky coincidence when Earth was hit by about 20 billion billion tonnes of asteroidal material," Willbold said in a statement.